Gas sensor control device

ABSTRACT

An O 2  sensor has a sensor element, which includes a solid electrolyte layer and a pair of electrodes, while the solid electrolyte layer is interposed between the electrodes. The O 2  sensor outputs an electromotive force signal in response to an air-to-fuel ratio of exhaust gas of an engine, which serves as a sensing subject. A constant current circuit, which induces a flow of a predetermined constant electric current between the pair of electrodes of a sensor element, and a current sensing arrangement, which senses a current value of an actual electric current that is conducted through the sensor element, are provided. A microcomputer determines whether an abnormality of the constant current circuit is present based on the current value of the electric current, which is sensed with the current sensing arrangement, in a case where the constant current is induced by the constant current circuit.

CROSS REFERENCE TO RELATED APPLICATION

The present application is the U.S. national phase of InternationalApplication No. PCT/JP2014/004014 filed on Jul. 31, 2014 and is based onand incorporates herein by reference Japanese Patent Application No.2013-167128 filed on Aug. 9, 2013.

The present application is based on and incorporates herein by referenceJapanese Patent Application No. 2013-167128 filed on Aug. 9, 2013.

TECHNICAL FIELD

The present disclosure relates to a gas sensor control device.

BACKGROUND ART

For instance, a gas sensor, which outputs an electromotive force, isprovided at a vehicle engine. In this type of gas sensor, exhaust gas,which is discharged from the engine, serves as a sensing subject of thegas sensor, and an oxygen concentration of the exhaust gas is sensedwith the gas sensor. This type of gas sensor includes an electromotiveforce cell, which outputs an electromotive force signal that variesdepending on whether the exhaust gas is rich or lean. Specifically, whenan air-to-fuel ratio is rich, the electromotive force cell outputs theelectromotive force signal of about 0.9 V. In contrast, when theair-to-fuel ratio is lean, the electromotive force cell outputs theelectromotive force signal of about 0 V.

In this type of gas sensor, when the air-to-fuel ratio of the exhaustgas changes between rich and lean, a change in the sensor output may bedisadvantageously delayed relative to an actual change in theair-to-fuel ratio. In order to improve the output characteristic of sucha gas sensor, various techniques have been proposed.

For instance, the Patent Literature 1 discloses a gas sensor controldevice, in which a constant current circuit is connected to at least oneof a pair of sensor electrodes. In this gas sensor control device, whenit is determined that a demand for changing the output characteristic ofthe gas sensor is present, a flow direction of the constant electriccurrent is determined based on the demand. Then, the constant currentcircuit is controlled to induce a flow of the constant electric currentin the determined direction. Through the supply of the constant electriccurrent, the output characteristic of the gas sensor is appropriatelycontrolled.

The air-to-fuel ratio control operation, which uses the sensed value ofthe gas sensor, significantly contributes to the reduction of theexhaust emissions of the engine. In a case where an abnormality ispresent in the gas sensor, the abnormality has a significant influencewith respect to the reduction of the exhaust emissions. Furthermore,even in a case where the gas sensor is normal, when the constantelectric current does not normally flow in the electromotive force cell,it may possibly have an influence on the exhaust emissions. With respectto the above point, it is desirable to correctly determine whether anabnormality is present in the constant current circuit.

CITATION LIST Patent Literature

-   PATENT LITERATURE 1: JP2012-63345A (corresponding to    US201210043205A1)

SUMMARY OF INVENTION

It is a major objective of the present disclosure to provide a gassensor control device that can appropriately sense an abnormality in aconstant current circuit, which induces a flow of a constant current ina gas sensor, and thereby to improve a reliability of a gas sensoroutput.

Hereinafter, means for solving the above objective and advantagesthereof will be described.

The present disclosure provides a gas sensor control device applied toan exhaust gas purifying device of an internal combustion engine, whichincludes: a catalyst that is installed in an exhaust device of theinternal combustion engine and purifies NOx, which is a lean componentof an exhaust gas of the internal combustion engine, and a richcomponent of the exhaust gas; and a gas sensor that is installed at alocation, which is in an intermediate portion of the catalyst or on adownstream side of the catalyst, to sense a gas component of the exhaustgas, which serves as a sensing subject, after purification of theexhaust gas with the catalyst, wherein the gas sensor includes anelectromotive force cell, which has a solid electrolyte body and a pairof electrodes, to output an electromotive force signal in response to anair-to-fuel ratio of the exhaust gas. The gas sensor control deviceincludes: a current conduction regulating device that induces a flow ofa predetermined constant electric current between the pair of electrodesof the electromotive force cell; a current sensing arrangement thatsenses a current value of an actual electric current, which is conductedthrough the electromotive force cell; and an abnormality determiningunit that determines whether an abnormality is present in the currentconduction regulating device based on the current value of the actualelectric current, which is sensed with the current sensing arrangement,in a case where the flow of the predetermined constant electric currentis induced by the current conduction regulating device.

The above construction is made in view of that even in the case wherethe gas sensor is normal, when the constant electric current does notnormally flow through the electromotive force cell, it may possibly havean influence on the exhaust emissions, and thereby the aboveconstruction is made to enable the appropriate determination of whetherthe abnormality is present in the current conduction regulating device(the constant current circuit). In this way, with respect to the gassensor, which uses the exhaust gas outflowing on the downstream side ofthe catalyst, i.e., the exhaust gas after the purification thereof atthe catalyst as the sensing subject, the reliability of the sensoroutput of the gas sensor can be improved. Particularly, in the casewhere the abnormality occurs in the current conduction regulating device(the constant current circuit), it is desirable that the occurrence ofthe abnormality can be detected in the early stage in order to minimizethe influence on the exhaust emissions. With respect to this point, inthe case where the actual current value of the electric current, whichis induced by the current conduction regulating device, is monitored,and the abnormality determination is made based on the actual currentvalue of the electric current, the abnormality determination can bealways made regardless of the operational state of the internalcombustion engine. Thereby, it is possible to implement the practicallydesirable construction.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram schematically showing an entire structure of anengine control system according to an embodiment of the presentdisclosure.

FIG. 2 is a diagram schematically showing a cross section of a sensorelement and a sensor control arrangement of the embodiment.

FIG. 3 is an electromotive force characteristic diagram indicating arelationship between an air-to-fuel ratio and an electromotive force ofthe sensor element.

FIG. 4 is a diagram showing catalytic conversion characteristics of afirst catalyst and output characteristics of an O₂ sensor.

FIG. 5 is a diagram showing catalytic conversion characteristics of thefirst catalyst and output characteristics of an O₂ sensor.

FIG. 6 is a schematic diagram for describing reactions of gas componentsat the sensor element.

FIG. 7 is a flowchart showing a procedure of a constant electric currentcontrol operation of the embodiment.

FIG. 8 is a diagram used for setting a command current value of aconstant current circuit in the embodiment.

FIG. 9 is a flowchart showing an abnormality determination process forthe constant current circuit.

FIG. 10 is a diagram showing a test result with respect to the amount ofNOx emissions and the amount of HC emissions on a downstream side of thefirst catalyst.

FIG. 11 is a diagram showing a structure of a sensor control arrangementin a modification of the embodiment.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present disclosure will be described with referenceto the accompanying drawings. In the present embodiment, a gas sensor,which is provided in an exhaust conduit of an engine (internalcombustion engine) of a vehicle, is used, and there will be described anengine control system, which executes various control operations of theengine based on an output of the gas sensor. In the control system, anelectronic control unit (hereinafter referred to as an ECU) is used toexecute, for example, a control operation of a fuel injection quantityand a control operation of ignition timing. FIG. 1 is a diagram thatschematically shows an entire structure of the system.

In FIG. 1, the engine 10 is, for example, a gasoline engine and has anelectronically controlled throttle valve 11, fuel injection valves 12,and ignition devices 13. Catalysts 15 a, 15 b, which serve as an exhaustgas purifying device, are installed in an exhaust conduit 14 (serving asan exhaust device) of the engine 10. Each of the catalysts 15 a, 15 b isformed as, for example, a three-way catalyst. The catalyst 15 a is afirst catalyst, which serves as an upstream side catalyst, and thecatalyst 15 b is a second catalyst, which serves as a downstream sidecatalyst. As is well known in the art, the three-way catalyst purifiesthree noxious components of the exhaust gas, i.e., CO (carbon monoxide),HC (hydrocarbon) and NOx (nitrogen oxide, such as NO) and is formed byapplying metal, such as platinum, palladium, rhodium, to a ceramicsubstrate that is configured into, for example, a honeycomb form or alattice form. In this instance, at the three-way catalyst, CO and HC,which are the rich components, are purified through an oxidationreaction, and NOx, which is the lean component, is purified through areduction reaction.

An A/F sensor 16 is placed on an upstream side of the first catalyst 15a, and an O₂ sensor (oxygen sensor) 17 is placed between the catalysts15 a, 15 b (on the downstream side of the first catalyst 15 a and on theupstream side of the second catalyst 15 b). The A/F sensor 16 outputs anA/F signal, which is generally proportional to the air-to-fuel ratio ofthe exhaust gas. Furthermore, the O₂ sensor 17 outputs an electromotiveforce signal, which varies depending on whether the air-to-fuel ratio ofthe exhaust gas is rich or lean.

Furthermore, the system is provided with various sensors, such as athrottle opening degree sensor 21, which senses the opening degree ofthe throttle valve 11, a crank angle sensor 22, which outputs a crankangle signal of a rectangular waveform at every predetermined crankangle (e.g., a period of 30 degree crank angle) of the engine, an airquantity sensor 23, which senses the quantity of the intake air drawninto the engine 10, and a coolant temperature sensor 24, which sensesthe temperature of the engine coolant. Although not depicted in thedrawings, besides the above sensors, there are also provided, forexample, a combustion pressure sensor, which senses a combustionpressure in a cylinder of the engine, an accelerator opening degreesensor, which senses an opening degree of an accelerator (an acceleratormanipulation amount), and an oil temperature sensor, which senses atemperature of an engine lubricating oil. These sensors respectivelyserve as an operational state sensing means.

The ECU 25 includes a microcomputer of a known type, which has, forexample, a CPU, a ROM, and a RAM. The ECU 25 executes various controlprograms, which are stored in the ROM, to perform various controloperations of the engine 10 according to the engine operational state.Specifically, the ECU 25 receives signals from the above-describedsensors, and the ECU 25 computes each corresponding fuel injectionquantity and each corresponding ignition timing to execute, for example,the control operation for driving the fuel injection valves 12 and thecontrol operation for driving the ignition devices 13 based on thesignals.

Particularly, with respect to the fuel injection quantity controloperation, the ECU 25 performs an air-to-fuel ratio feedback controloperation based on a measurement signal of the A/F sensor 16, which isplaced on the upstream side of the first catalyst, and a measurementsignal of the O₂ sensor 17, which is placed on the downstream side ofthe first catalyst. Specifically, the ECU 25 executes a main feedbackcontrol operation in such a manner that an actual air-to-fuel ratio (anactual air-to-fuel ratio at the location on the upstream side of thefirst catalyst), which is sensed with the A/F sensor 16, coincides witha target air-to-fuel ratio, which is set based on the engine operationalstate. Also, the ECU 25 executes a sub-feedback control operation insuch a manner that an actual air-to-fuel ratio (an actual air-to-fuelratio at the location on the downstream side of the first catalyst),which is sensed with the O₂ sensor 17, coincides with a targetair-to-fuel ratio. In the sub-feedback control operation, in view of,for example, a difference between the actual air-to-fuel ratio on thedownstream side of the first catalyst and the target air-to-fuel ratio,the target air-to-fuel ratio used in the main feedback control operationis corrected, or a feedback correction amount used in the main feedbackcontrol operation is corrected. The ECU 25 executes a stoichiometricfeedback control operation, which sets the target air-to-fuel ratio to astoichiometric air-to-fuel ratio (theoretical air-to-fuel ratio), as theair-to-fuel ratio control operation.

Next, the structure of the O₂ sensor 17, which is placed on thedownstream side of the first catalyst, will be described. The O₂ sensor17 has a sensor element 31, which is configured into a cup shape. FIG. 2shows a cross section of the sensor element 31. In reality, the sensorelement 31 is configured such that the entire sensor element 31 isreceived in a housing or an element cover, and the sensor element 31 isplaced in the exhaust conduit 14. The sensor element 31 serves as anelectromotive force cell.

In the sensor element 31, a solid electrolyte layer 32, which serves asa solid electrolyte body, has a cup shaped cross section. An exhaustside electrode 33 is formed in an outer surface of the solid electrolytelayer 32, and an atmosphere side electrode 34 is formed in an innersurface of the solid electrolyte layer 32. Each of the electrodes 33, 34is formed as a layer on the corresponding surface of the solidelectrolyte layer 32. The solid electrolyte layer 32 is an oxidesintered body, which conducts oxygen ions therethrough and is formed bycompletely dissolving CaO, MgO, Y₂O₃, and/or Yb₂O₃ as stabilizer intoZrO₂, HfO₂, ThO₂, and/or Bi₂O₃. Furthermore, each electrode 33, 34 ismade of a noble metal, such as platinum, which has the high catalyticactivity, and a surface of the electrode 33, 34 is covered with a porouscoating that is chemically plated. The above-described electrodes 33, 34serve as a pair of electrodes (sensor electrodes). An inside space,which is surrounded by the solid electrolyte layer 32, is an atmospherechamber (a reference gas chamber) 35. A heater 36 is received in theatmosphere chamber 35. The heater 36 has a sufficient heat capacity toactivate the sensor element 31, and the sensor element 31 is entirelyheated by a heat energy, which is generated from the heater 36. Anactivation temperature of the O₂ sensor 17 is, for example, 500 to 650degrees Celsius. The atmosphere gas is introduced into the atmospherechamber 35, so that the inside of the atmosphere chamber 35 ismaintained at a predetermined oxygen concentration.

In the sensor element 31, the exhaust gas is present at the outside (theelectrode 33 side) of the solid electrolyte layer 32, and the atmospheregas is present at the inside (the electrode 34 side) of the solidelectrolyte layer 32. An electromotive force is generated between theelectrode 33 and the electrode 34 in response to a difference in anoxygen concentration (a difference in an oxygen partial pressure)between the outside (the electrode 33 side) of the solid electrolytelayer 32 and the inside (the electrode 34 side) of the solid electrolytelayer 32. Specifically, the generated electromotive force variesdepending on whether the air-to-fuel ratio is rich or lean. In such acase, the oxygen concentration at the exhaust side electrode 33 is lowerthan the oxygen concentration at the atmosphere side electrode 34, whichserves as a reference side electrode, and the electromotive force isgenerated at the sensor element 31 while the atmosphere side electrode34 and the exhaust side electrode 33 serve as a positive side and anegative side, respectively. Thus, the O₂ sensor 17 outputs theelectromotive force signal, which corresponds to the oxygenconcentration (the air-to-fuel ratio) of the exhaust gas.

FIG. 3 is an electromotive force characteristic diagram showing arelationship between the air-to-fuel ratio of the exhaust gas and theelectromotive force of the sensor element 31. In FIG. 3, the axis ofabscissas indicates a percentage of excess air λ. When the percentage ofexcess air λ is 1 (i.e., λ=1), the air-to-fuel ratio is thestoichiometric air-to-fuel ratio (theoretical air-to-fuel ratio). Thesensor element 31 has the characteristics of that the electromotiveforce generated from the sensor element 31 varies depending on whetherthe air-to-fuel ratio is rich or lean, and the electromotive forcegenerated from the sensor element 31 rapidly changes around thestoichiometric air-to-fuel ratio. Specifically, the electromotive forceof the sensor element 31 at the rich time is about 0.9 V, and theelectromotive force of the sensor element 31 at the lean time is about 0V.

In FIG. 2, a sensor control arrangement 40 is connected to the sensorelement 31. When the electromotive force is generated at the sensorelement 31 in response to the air-to-fuel ratio (the oxygenconcentration) of the exhaust gas, the sensor measurement signal (theelectromotive force signal), which corresponds to the electromotiveforce generated at the sensor element 31, is outputted from the sensorelement 31 to a microcomputer 41 of the sensor control arrangement 40.The microcomputer 41 computes the air-to-fuel ratio based on theelectromotive force signal of the sensor element 31. The sensor controlarrangement 40 is formed in the ECU 25 of FIG. 1. At the ECU 25, themicrocomputer 41 is formed as a computing means that has an enginecontrol function and a sensor control function. In this case, themicrocomputer 41 computes the engine rotational speed and the intake airquantity based on the measurement results of the various sensorsdiscussed above. However, the ECU 25 may be constructed to have anengine control microcomputer, which executes the engine controlfunction, and a sensor control microcomputer, which executes the sensorcontrol function, if desired.

Furthermore, the microcomputer 41 determines an activated state of thesensor element 31 and controls the driving operation of the heater 36through a drive device 42, which is connected to the heater 36 throughan electric path 50 c, based on a result of determination of theactivated state of the sensor element 31. The technique of theactivation determination of the sensor element 31 and the technique ofthe heater control are already known. Therefore, the activationdetermination of the sensor element 31 and the heater control will bebriefly described. The microcomputer 41 periodically changes the voltageor the electric current applied to the sensor element 31 in a mannerthat is similar to an alternating current and senses a thus generatedchange in the electric current or a thus generated change in theelectric voltage. A resistance of the sensor element 31 (an impedance ofthe sensor element 31) is computed based on the change in the electriccurrent or the change in the voltage, and the energization controloperation of the heater 36 is executed based on the resistance of thesensor element 31. At that time, there is a correlation between theactivated state of the sensor element 31 (the temperature of the sensorelement 31) and the resistance of the sensor element 31. When theresistance of the sensor element 31 is controlled to a predeterminedtarget value, the sensor element 31 is held in the desired activatedstate (the state, under which the activation temperature of the sensorelement 31 is held in a range of 500 to 650 degrees Celsius). Forexample, a sensor element temperature feedback control operation may beexecuted as the heater control operation.

When the engine 10 is operated, the actual air-to-fuel ratio of theexhaust gas is changed. For example, the air-to-fuel ratio may berepeatedly changed between rich and lean. At the time of changing theactual air-to-fuel ratio between rich and lean, when a deviation existsbetween the output of the O₂ sensor 17 and the presence of NOx, which isthe lean component, the emission performance may possibly be influenced.For example, the amount of NOx in the exhaust gas may possibly beincreased beyond the intended amount at the time of operating the engine10 under the high load (the time of accelerating the vehicle).

In the present embodiment, a sensing mode of the O₂ sensor 17 is changedbased on the relationship between the output characteristic of the O₂sensor 17, which outputs the electromotive force, and the catalyticconversion characteristic of the first catalyst 15 a, which is placed onthe upstream side of the O₂ sensor 17. Details of the change of thesensing mode of the O₂ sensor 17 will be described later. FIG. 4 is adiagram that shows the catalytic conversion characteristics of the firstcatalyst 15 a, which is the three-way catalyst, and the outputcharacteristics of the O₂ sensor 17. Specifically, FIG. 4 shows: arelationship between a catalytic conversion efficiency of each of thethree noxious components (i.e., CO, HC, NOx) of the exhaust gas at thefirst catalyst 15 a and the air-to-fuel ratio; a relationship betweenthe gas concentration of each of the three noxious components and theoxygen on the downstream side of the first catalyst 15 a and theair-to-fuel ratio; a relationship between the gas concentration of eachof the three noxious components and the oxygen around the surface of theexhaust side electrode 33 of the O₂ sensor 17 and the air-to-fuel ratio;and a relationship between the electromotive force output of the O₂sensor 17 and the air-to-fuel ratio.

The first catalyst (the three-way catalyst) 15 a has a catalyticconversion window, in which the catalytic conversion efficiency of eachof the three noxious components becomes high around the point of thestoichiometric air-to-fuel ratio (percentage of excess air λ=1), as isknown in the art. Furthermore, with respect to the concentrations of thethree noxious components and the concentration of the oxygen on thedownstream side of the first catalyst 15 a, it is understood that areaction equilibrium point A1, at which the concentrations of the richcomponents (CO, HC) and the concentration of the oxygen become generallyequal to one another, is present around the point of the stoichiometricair-to-fuel ratio, and an NOx outflow point A2, at which NOx (NO) beginsto outflow from the first catalyst 15 a on the downstream side of thefirst catalyst 15 a, is also present. In this case, it is understoodthat the NOx outflow point A2 (the point of starting the outflow of NOxfrom the catalyst 15 a) is located on the rich side of the reactionequilibrium point A1, and the NOx outflow point A2 and the reactionequilibrium point A1 are spaced from each other by a difference ΔA. Thatis, the first catalyst 15 a has the catalytic conversion characteristicof that the NOx outflow point (serving as a second air-to-fuel ratiopoint) A2, at which NOx begins to outflow from the first catalyst 15 a,is located on the rich side of the reaction equilibrium point (servingas a first air-to-fuel ratio point) A1, which forms the equilibriumpoint for the rich components and the oxygen. It could be said that thereaction equilibrium point A1 is an inflection point of the equilibriumcharacteristic of the rich components and the oxygen, and the NOxoutflow point A2 is an inflection point of the outflow concentrationcharacteristic of NOx.

The reason for the generation of the deviation between the point A1 andthe point A2 may be as follows. In the case where the exhaust gas, whichcontains CO, HC, NOx, and O₂, is guided to the first catalyst 15 aduring the operation of the engine 10, NOx may possibly outflow from thefirst catalyst 15 a in addition to CO and HC. For example, even in therange of the catalytic conversion window of the three-way catalyst, itwill be noted that some amount of CO, HC, and NOx outflows from thefirst catalyst 15 a when the amount of CO, HC, and NOx is preciselymeasured. In such a case, although O₂ outflows from the first catalyst15 a in equilibrium with CO and HC (starting of the outflow of O₂ at theconcentration of CO and HC≈0), NOx outflows from the first catalyst 15 aon the downstream side thereof regardless of the reaction of CO and HC.Therefore, the deviation exists between the point A1 and the point A2.

Furthermore, the concentrations of the above three components and theoxygen around the exhaust side electrode of the O₂ sensor 17 are thesame as the concentrations of the above three components and the oxygenon the downstream side of the first catalyst 15 a. In this case, theamount of the rich components (CO, HC) is larger than the amount ofoxygen on the rich side of the point A1, and the amount of oxygen islarger than the amount of the rich components on the lean side of thepoint A1. Therefore, in terms of the electromotive force of the O₂sensor 17, one of a rich signal (0.9 V) and a lean signal (0 V) isoutputted on one side or the other side of the reaction equilibriumpoint A1 of the first catalyst 15 a. In this case, it can be said thatthe reaction equilibrium point for the rich components and the oxygen atthe O₂ sensor 17 coincides with the reaction equilibrium point A1 at thefirst catalyst 15 a. Furthermore, NOx is present on the rich side of thepoint A1.

At the exhaust side electrode of the O₂ sensor 17, the oxidationreaction and the reduction reaction of CO, HC and NOx of the exhaust gastake place according to the following chemical reaction formulae (1) to(3).CO+0.5O₂→CO₂   (1)CH₄+2O₂→CO₂+2H₂O   (2)CO+NO→CO₂+0.5N₂   (3)Furthermore, there is established a relationship of k1, k2>>K3 where k1,k2 and k3 denote an equilibrium constant of the chemical reactionformula (1), an equilibrium constant of the chemical reaction formula(2), and an equilibrium constant of the chemical reaction formula (3),respectively.

In this case, at the O₂ sensor 17, the equilibrium point (the point atwhich the electromotive force output=0.45 V) is determined through thegas reactions of, for example, CO, HC NOx, and O₂. However, due to thedifferences in the equilibrium constant, the reactions of CO and HC withO₂ become main reactions at the exhaust side electrode 33.

Furthermore, the above difference ΔA is present in the catalyticconversion characteristic of the first catalyst 15 a, and the abovedifference ΔA has the influence on the output characteristic of the O₂sensor 17. Therefore, in some cases, even when NOx outflows from thefirst catalyst 15 a, the output of the O₂ sensor 17 may not correspondto the outflow of NOx from the first catalyst 15 a. Thus, the outflow ofNOx from the first catalyst 15 a cannot be correctly monitored, andthereby the amount of NOx emissions may possibly be increased.

In view of the above disadvantage, according to the present embodiment,the electric current, which has a predetermined current value, isconducted between the electrodes 33, 34 of the sensor element 31 of theO₂ sensor 17, so that at the location around the exhaust side electrodeof the O₂ sensor 17, the concentrations of the rich components arereduced, and the concentration of the oxygen is increased. Specifically,as shown in FIG. 5, the equilibrium point of the gas reaction around theexhaust side electrode of the O₂ sensor 17 is changed from the point A1to a point A3. In FIG. 5, in comparison to FIG. 4, all of theconcentration characteristics of CO, HC and O₂ around the exhaust sideelectrode of the O₂ sensor 17 are shifted to the rich side. In this way,in the case where the output characteristic of the O₂ sensor 17 ischanged, and NOx outflows from the first catalyst 15 a, the output ofthe O₂ sensor 17 can correspond to the outflow of NOx.

The principle of inducing the change in the sensor output characteristicthrough conduction of the electric current between the electrodes 33, 34is as follows. As shown in FIG. 6, CO, HC, NOx and O₂ are present aroundthe exhaust side electrode 33 of the O₂ sensor 17. Under such acircumstance, the electric current is conducted through the sensorelement 31 such that the oxygen ions are moved from the atmosphere sideelectrode 34 to the exhaust side electrode 33 through the solidelectrolyte layer 32. Specifically, the oxygen pumping is executed atthe sensor element 31. In this case, at the exhaust side electrode 33,the oxygen, which is moved to the exhaust side electrode 33 side throughthe solid electrolyte layer 32, reacts with CO and HC to form CO₂ andH₂O, respectively. In this way, CO and HC are removed around the exhaustside electrode 33, and the equilibrium point of the gas reaction aroundthe exhaust side electrode 33 of the O₂ sensor 17 is shifted to the richside.

Next, the structure of the sensor control arrangement 40, which executesthe control operation of the O₂ sensor 17, will be described. Thestructure of the sensor control arrangement 40 is one shown in FIG. 2.Specifically, the sensor control arrangement 40 includes themicrocomputer 41, which serves as a control device (control means). Themicrocomputer 41 obtains the electromotive force signal, which isoutputted from the sensor element 31, through, for example, an A/Dconverter, and the microcomputer 41 computes the air-to-fuel ratio(particularly, the air-to-fuel ratio on the downstream side of the firstcatalyst) of the exhaust gas based on the obtained electromotive forcesignal. Furthermore, a constant current circuit 43, which serves as acurrent conduction regulating device (current conduction regulatingmeans), is connected in an electric path 50 a, which electricallyconnects between the atmosphere side electrode 34 of the sensor element31 and the microcomputer 41, through an electric path 80. The constantcurrent circuit 43 is configured such that when the sensor element 31generates the electromotive force, the constant current circuit 43induces a flow of a predetermined constant electric current through thesensor element 31.

The constant current circuit 43 is configured to enable the flow of theconstant electric current Ics from the exhaust side electrode 33 to theatmosphere side electrode 34 through the solid electrolyte layer 32 inthe sensor element 31. Furthermore, the constant current circuit 43 hasa PWM drive device, and thereby the constant current circuit 43 canadjust a current value of the electric current through a PWM controloperation (a duty control operation). The microcomputer 41 sets thecurrent value of the constant electric current of the constant currentcircuit 43 (the current value of the electric current conducted throughthe constant current circuit 43) based on a demand for conducting theelectric current and controls the constant current circuit 43 to inducethe flow of the constant electric current Ics, which has the set currentvalue. Here, it should be noted that the term “current value” refers toa value of the current that is expressed in units of, for example,amperes (A).

In the present embodiment, the control operation of the constantelectric current is executed based on a difference between the reactionequilibrium point A1 of the oxygen outflow at the first catalyst 15 aand the NOx outflow point A2 of the NOx outflow at the first catalyst 15a. Particularly, the constant electric current is controlled such thatthe equilibrium point of the gas reaction around the exhaust sideelectrode of the O₂ sensor 17 is placed at the NOx outflow point A2 or apoint adjacent to the NOx outflow point A2. In this way, the outputcharacteristic of the O₂ sensor 17 is changed based on the catalyticconversion characteristic of the first catalyst 15 a. Thereby, when NOxoutflows from the first catalyst 15 a, the lean signal is outputted atthe O₂ sensor 17 from the beginning of the outflow of NOx from the firstcatalyst 15 a.

Here, in view of ensuring the robustness of the O₂ sensor 17 for thepurpose of limiting the NOx emissions, it is desirable that theequilibrium point of the gas reaction around the exhaust side electrodeof the O₂ sensor 17 is placed on the rich side of the NOx outflow pointA2 (see FIG. 5). Specifically, the equilibrium point of the gas reactionaround the exhaust side electrode of the O₂ sensor 17 may be shiftedfrom the NOx outflow point A2 on the rich side of the NOx outflow pointA2 by the amount of, for example, about 0.1 to 0.5% (more desirably 0.1to 0.3%) in terms of the percentage of excess air λ to have a slightlyrich state.

When the operational state of the engine 10 is changed, the amount ofthe rich components in the exhaust gas is changed. Specifically, whenthe rotational speed of the engine is increased, or when the load of theengine is increased, the amount of the rich components in the exhaustgas is increased. In other words, when the rotational speed or the loadof the engine is increased, the flow rate of the rich gas is increased,and the gas concentration of the rich gas is increased. In such a case,when the current value of the electric current to be supplied to thesensor element 31 is kept constant regardless of the engine operationalstate, the equilibrium point of the gas reaction around the exhaust sideelectrode of the O₂ sensor 17 may possibly be unintentionally deviatedfrom a desirable position, which is set with reference to the NOxoutflow point A2. That is, the amount of supplied oxygen, which issupplied around the exhaust side electrode of the sensor element 31 bythe supply of the electric current through the sensor element 31, maypossibly fall short relative to the amount of the rich components aroundthe exhaust side electrode of the sensor element 31. When this shortageof the supplied oxygen occurs, the rich components remain around theexhaust side electrode 33. Thereby, the output characteristic of the O₂sensor 17 cannot be changed in the desirable manner.

Therefore, in the present embodiment, the current value of the electriccurrent conducted through the sensor element 31 (the current value ofthe electric current of the constant current circuit 43) is variablycontrolled based on the operational state of the engine 10. In thiscase, even when the amount of the required oxygen, which is required tohave the equilibrium reaction of the rich gas on the surface of theexhaust side electrode at the O₂ sensor 17, is changed in response tothe engine operational state, the output characteristic of the O₂ sensor17 can be changed in the desirable manner in response to the change inthe amount of the required oxygen. An engine rotational speed, an engineload and/or a load rate of the engine may be used as a parameter(s) ofthe engine operational state.

Furthermore, in the case where the output characteristic of the O₂sensor 17 is changed by the current value of the electric current of theconstant current circuit 43, as discussed above, when an abnormalityoccurs in the constant current circuit 43, the exhaust emissionperformance is influenced. Therefore, in the present embodiment, anabnormality determining function, which executes abnormalitydetermination of a determination subject, i.e., the constant currentcircuit 43, is added to the microcomputer 41.

As shown in FIG. 2, as a structure that is used to sense theabnormality, a shunt resistor 45 for sensing the electric current isconnected to the exhaust side electrode 33, and the electric current,which flows through the shunt resistor 45, is sensed with a currentsensing device 46. Specifically, the shunt resistor 45 is installed inthe electric path 50 b, which connects between the exhaust sideelectrode 33 and the ground. The current sensing device 46 is connectedto the exhaust side electrode 33 of the shunt resistor 45 and theground, and the current, which flows through the shunt resistor 45, issensed with the current sensing device 46. The current sensing device 46may include a differential amplifier circuit, which has, for example, anoperational amplifier. In this case, the actual current value of theelectric current, which is conducted by the constant current circuit 43,is sensed with the shunt resistor 45 and the current sensing device 46,and the microcomputer 41 executes the abnormality determination fordetermining whether the abnormality is present in the constant currentcircuit 43 based on the actual current value of the electric current.The shunt resistor 45 and the current sensing device 46 cooperate witheach other and serve as a current sensing arrangement of the presentdisclosure.

Next, a constant current control operation and an abnormalitydetermination process, which are executed by the microcomputer 41, willbe described with reference to flowcharts. FIG. 7 is the flowchartshowing the constant current control operation. This operation isrepeated by the microcomputer 41 at predetermined time intervals.

In FIG. 7, at step S11, it is determined whether an execution conditionfor executing the constant current control operation is satisfied. Forinstance, the execution condition may include the followings: the O₂sensor 17 and the constant current circuit 43 are both normal; and thesub-feedback control operation is currently executed. When the answer tothe inquiry at step S11 is YES, the operation proceeds to step S12.

At step S12, the engine operational state, such as the engine rotationalspeed and/or the engine load (e.g., the amount of the intake air), isobtained. Thereafter, at next step S13, a command current value of theelectric current is set based on the engine operational state, which isobtained at step S12. At this time, the command current value of theelectric current is set based on, for example, a relationship shown inFIG. 8. In FIG. 8, when the engine rotational speed or the engine loadis increased, the command current value of the electric current isincreased.

Thereafter, at step S14, the control (control for conduction of electriccurrent) of the constant current circuit 43 is executed such that theconstant current circuit 43 induces the flow of the constant electriccurrent, which has the current value set at step S13, through the sensorelement 31.

FIG. 9 shows the flowchart indicating the abnormality determinationprocess of the constant current circuit 43. This process is repeated atpredetermined time intervals by the microcomputer 41.

In FIG. 9, at step S21, it is determined whether an execution conditionfor executing the abnormality determination process is satisfied. Forexample, this execution condition may include a condition of that thesensor element 31 is in the activated state, i.e., the temperature ofthe sensor element 31 is equal to or higher than the predeterminedactivation temperature. When the answer to the inquiry at step S21 isYES, the operation proceeds to step S22.

At step S22, the command current value of the electric current for theconstant current circuit 43 at this time is obtained, and an actualcurrent value of the electric current, which is sensed with the shuntresistor 45 and the current sensing device 46 at this time, is obtained.At this time, in a case where the command current value has been changedimmediately before the time of executing step S22, the actual currentvalue of the electric current should be sensed with the shunt resistor45 and the current sensing device 46 and is obtained by themicrocomputer 41 from the current sensing device 46 after stabilizationof the actual current value of the electric current. Specifically, theactual current value is sensed and is obtained by the microcomputer 41after elapsing of a predetermined time period from the time of changingthe command current value.

Then, at step S23, the command current value and the actual currentvalue are compared with each other, and it is determined whether adifference (absolute value of the difference) between the commandcurrent value and the actual current value is smaller than apredetermined determination value K. When it is determined that thedifference (absolute value of the difference) between the commandcurrent value and the actual current value is smaller than thedetermination value K at step S23, the operation proceeds to step S24where it is determined that the constant current circuit 43 is normal.In contrast, when it is determined that the difference (absolute valueof the difference) between the command current value and the actualcurrent value is equal to or larger than the determination value K atstep S23, the operation proceeds to step S25 where it is determined thatthe constant current circuit 43 is abnormal. Desirably, thedetermination value K is set in view of a circuit tolerance (e.g., atolerance of a sensor IC). When it is determined that the constantcurrent circuit 43 is abnormal at step S25, the operation proceeds tostep S26. At step S26, there is executed a fail-safe operation, such asstopping of the conduction (supply) of the constant electric currentthrough the constant current circuit 43, stopping of the sub-feedbackcontrol operation of the air-to-fuel ratio, turning on of an abnormalitywarning lamp provided in, for example, an instrument panel, and/orstoring of diagnosis data in a storage device.

The microcomputer 41 includes a setting unit, which has a function ofsetting the command current value by executing steps S11-S13 of theflowchart of FIG. 7, and an abnormality determining unit, which has afunction of determining whether the abnormality is present in theconstant current circuit 43 by executing steps S21-S25 of the flowchartof FIG. 9. It should be noted that these functions are not necessarilyexecuted by the single microcomputer and may be executed by a pluralityof microcomputers.

The present embodiment discussed above provides the followingadvantages.

Even in the case where the O₂ sensor 17 is normal, when the constantelectric current does not normally flow through the sensor element 31,it may possibly have an influence on the exhaust emissions. The presentembodiment is implemented in view of this point, and there is providedthe construction that enables the appropriate determination of whetherthe abnormality is present in the constant current circuit 43. In thisway, the reliability of the output of the O₂ sensor can be improved.Particularly, in the case where the abnormality is present in theconstant current circuit 43, it is desirable that the presence of theabnormality is detected at an early stage. With respect to this point,the construction, which monitors the actual current value of theelectric current induced by the constant current circuit 43 and executesthe abnormality determination based on the actual current value of theelectric current, can always execute the abnormality determinationregardless of, for example, the operational state of the engine 10.Therefore, it is possible to implement the construction that ispractically desirable.

The current value (the command current value) of the electric currentinduced by the constant current circuit 43 is controlled based on theengine operational state. In this way, even when the amount of the richcomponents in the exhaust gas is changed due to the change in the engineoperational state, the output characteristic of the O₂ sensor 17 can beappropriately changed, and thereby the equilibrium point of the gasreaction around the exhaust side electrode of the O₂ sensor 17 can bemaintained at the desired position relative to the NOx outflow point A2.

In the case where the command current value of the electric current isset based on the engine operational state, the abnormalitydetermination, which determines whether the abnormality is present inthe constant current circuit 43, is executed based on the result of thecomparison between the command current value of the electric current andthe actual current value of the electric current. In this way, even whenthe command current value of the electric current is changed in responseto the engine operational state, the appropriate abnormalitydetermination can be executed.

In the state where the sensor element 31 is in the low temperature state(the deactivated state), the resistance of the sensor element 31 ishigh. In such a state, even though the constant electric current shouldbe normally conducted through the sensor element 31, the actual electriccurrent does not substantially flow through the sensor element 31. Withrespect to this point, the abnormality determination is executed uponsatisfaction of the condition of that the temperature of the sensorelement 31 is equal to or higher than the predetermined temperature (thepredetermined activation temperature). Therefore, the abnormalitydetermination, which is highly reliable, can be executed.

Through use of the constant current circuit 43, which is constructed inthe above-described manner, the output characteristic of the O₂ sensor17 can be adjusted to correspond with the air-to-fuel ratio at the pointwhere the outflow of NOx begins at the first catalyst 15 a. That is, inthe case where the NOx outflows from the first catalyst 15 a, theelectromotive force output of the O₂ sensor 17, which corresponds tothis outflow, is generated. Therefore, the output characteristic of theO₂ sensor 17 can be appropriately changed, and thereby the NOx emissionscan be limited. Furthermore, when the abnormality determination of theconstant current circuit 43 is executed in the above-described manner,the reliability of the sensor output characteristic can be improved atthe time of adjusting the sensor output characteristic.

The constant electric current Ics, which is conducted by the constantcurrent circuit 43, shifts the equilibrium point of the gas reactionaround the exhaust side electrode of the O₂ sensor 17 to the NOx outflowpoint A2 (the second air-to-fuel ratio point) or the point adjacent tothe NOx outflow point A2. Thereby, there is implemented the constructionthat is more appropriate in terms of limiting the NOx emissions throughuse of the output of the O₂ sensor 17.

Particularly, when the constant electric current Ics is supplied throughthe constant current circuit 43 in such a manner that the equilibriumpoint of the gas reaction around the exhaust side electrode of the O₂sensor 17 becomes slightly rich relative to the NOx outflow point A2(the second air-to-fuel ratio point), the required robustness can beachieved to limit the NOx emissions.

FIG. 10 indicates a test result with respect to the amount of emissionsof NOx and the amount of emissions of HC on the downstream side of thefirst catalyst 15 a. In FIG. 10, there is indicated the result of theair-to-fuel ratio control operation executed in the state where theoutput characteristic of the O₂ sensor 17 is changed. In FIG. 10, “A1”indicates the reaction equilibrium point A1 at the first catalyst 15 a,and “A2” indicates the NOx outflow point A2, and “λ shift amount”indicates a rich change amount in terms of A (percentage of excess air).In this case, it should be understood that the amount of NOx emissionsis reduced in a range where the λ shift amount is 0 to 0.5%.Furthermore, an appropriate value of the λ shift amount is 0.1 to 0.5%.Desirably, the current value of the electric current to be conductedthrough the sensor element 31 is a value in a rage of 0.1 mA to 1.0 mA.

(Other Embodiments)

The present disclosure is not necessarily limited to the aboveembodiment, and, for example, the above embodiment may be modified asfollows.

The current conduction regulating device may be constructed to have astructure shown in FIG. 11. In FIG. 11, there is provided a constantcurrent circuit 50 as the current conduction regulating device. When thesensor electromotive force is applied to the constant current circuit50, the constant current circuit 50 conducts the constant electriccurrent, which corresponds to this electromotive force. The constantcurrent circuit 50 includes a voltage generating arrangement 51, whichgenerates a predetermined constant voltage, an operational amplifier 52,an n-channel MOSFET 53, which is driven by an output of the operationalamplifier 52, and a resistor 54, which is connected to a source of theMOSFET 53. The MOSFET 53 and the resistor 54 are connected in series inan electric path 80, which connects between the electric path 50 a andthe ground. The voltage generating arrangement 51 includes a constantvoltage source 51 a (e.g., 5 V) and two resistors 51 b, 51 c. Theconstant voltage source 51 a and the resistors 51 b, 51 c are connectedin series, and an intermediate point between the resistors 51 b, 51 cforms a voltage output point X1. A + input terminal of the operationalamplifier 52 is connected to the voltage output point X1, and an outputterminal of the operational amplifier 52 is connected to a gate of theMOSFET 53. Furthermore, a − input terminal of the operational amplifier52 is connected to an intermediate point X2 between the MOSFET 53 andthe resistor 54. The gate, the drain and the source of the MOSFET 53 areconnected to the output terminal of the operational amplifier 52, theatmosphere side electrode 34 of the sensor element 31 and the resistor54, respectively.

The constant current circuit 50, which is constructed in theabove-described manner, is operated such that the voltage of the + inputterminal of the operational amplifier 52 and the voltage of the −terminal of the operational amplifier 52 become equal to each other.Therefore, the voltage of the intermediate point X2 and the voltage ofthe voltage output point X1 are equal to each other. The constantelectric current Ics, which has a current value determined based on thevoltage of the intermediate point X2 and the resistance value of theresistor 54, flows through a series circuit that includes the sensorelement 31, the MOSFET 53 and the resistor 54. At this time, the MOSFET53 is operated in response to an output voltage of the operationalamplifier, which is generated due to a difference between the inputvoltage of the + terminal and the input voltage of the − terminal of theoperational amplifier, so that the MOSFET 53 functions as a currentcontrol element that conducts the constant electric current Ics.

Desirably, the voltage of the voltage output point X1, the voltage ofthe intermediate point X2 and the resistance value of the resistor 54are determined based on the current value of the electric current thatneeds to be conducted through the sensor element 31 at the time ofgenerating the electromotive force in the sensor element 31.Specifically, in a case where the electric current, which has thecurrent value of 0.1 mA, needs to be conducted through the sensorelement 31 at the time of generating the electromotive force (0 to 0.9V) in the sensor element 31, the voltage of the voltage output point X1and the voltage of the intermediate point X2 are set to be 10 mV, andthe resistance value of the resistor 54 is set to be 100Ω. Furthermore,in a case where the electric current, which has the current value of 0.2mA, needs to be conducted through the sensor element 31, the voltage ofthe voltage output point X1 and the voltage of the intermediate point X2are set to be 20 mV, and the resistance value of the resistor 54 is setto be 100Ω. In addition, in a case where the electric current, which hasthe current value in a range of 0.1 mA to 1.0 mA, needs to be conductedthrough the sensor element 31, when the resistance value of the resistor54 is set to be 100Ω, the voltage of the voltage output point X1 and thevoltage of the intermediate point X2 may be set to be in a range of 10mV to 100 mV. However, in such a case, the reference voltage, which isthe voltage of the intermediate point X2 between the MOSFET 53 and theresistor 54 in the constant current circuit 50, is smaller than theelectromotive force (0.45 V) of the sensor element 31 generated at thestoichiometric air-to-fuel ratio.

Desirably, the range of the resistance value of the resistor 54 is about50Ω to 500Ω.

In the sensor control arrangement 40, which has the constant currentcircuit 50 of the above structure, when the electromotive force isgenerated in the sensor element 31, the predetermined constant electriccurrent Ics is conducted through the MOSFET 53 and the resistor 54 whileusing the electromotive force of the sensor element 31 as an electricpower source (in other words, the sensor element 31 being used as abattery). Thereby, the output characteristic of the O₂ sensor 17 can bechanged.

Furthermore, the microcomputer 41 changes, for example, the voltagevalue (the voltage value at the point X1) of the constant voltage of thevoltage generating arrangement 51 based on the engine operational state.In this way, the current value of the electric current conducted throughthe sensor element 31 (the current value of the electric currentconducted through the constant current circuit 50) is variablycontrolled. In this case, similar to the above embodiment, even when theamount of required oxygen, which is required to have the equilibriumreaction of the rich gas on the surface of the exhaust side electrode 33at the O₂ sensor 17, is changed in response to the engine operationalstate, the output characteristic of the O₂ sensor 17 can be changed in adesirable manner in response to the change in the required oxygen.

In the structure of FIG. 11, the actual current value of the electriccurrent, which is conducted through the sensor element 31, may besensed, and the abnormality determination, which determines whether theabnormality is present in the constant current circuit 50, may beexecuted based on the actual current value of the electric current andthe command current value of the electric current (the current value ofthe constant electric current that corresponds to the voltage value atthe point X1). The abnormality determination process may be executed byusing the above-described construction (FIG. 9), and thereby thedescription of the abnormality determination process is omitted.

A voltage circuit, which applies a positive voltage to the exhaust sideelectrode 33, may be provided between the exhaust side electrode 33 ofthe sensor element 31 and the ground. This voltage circuit may be anoffset voltage circuit that raises an electric potential of the exhaustside electrode 33 by a predetermined electric potential relative to anelectric potential of the output side of the constant current circuit43, from which the electric current flows out of the constant currentcircuit 43. In such a case, the electromotive force outputted from thesensor element 31 has a voltage that is obtained by adding thepredetermined offset voltage to the voltage of the original sensorelectromotive force (0 to 0.9 V) of the sensor element 31. Thisconstruction is particularly advantageous in a case where the sensorelement 31 is used as the battery as in the case of FIG. 11. Forinstance, in the sensor control arrangement 40 of FIG. 11, in the casewhere the electromotive force of the sensor element 31 changes in therange of 0 to 0.9 V, when the electromotive force becomes small, thecurrent value of the electric current, which is induced by the constantcurrent circuit 50 and is conducted through the sensor element 31, maypossibly become smaller than the proper current value. In view of thispoint, by installing the voltage circuit between the exhaust sideelectrode 33 and the ground, even in the range where the electromotiveforce is relatively small, it is possible to limit the excessivereduction of the current value of the electric current, which is inducedby the constant current circuit 50 and is conducted through the sensorelement 31. Thereby, the output characteristic of the O₂ sensor 17 canbe appropriately changed.

In the above embodiment (see FIG. 2), the constant current circuit 43 isconnected to the atmosphere side electrode 34 among the pair ofelectrodes 33, 34 of the sensor element 31. This construction may bemodified. The constant current circuit 43 may be connected to theexhaust side electrode 33. Alternatively, the constant current circuit43 may be provided to both of the electrodes 33, 34. In the aboveembodiment, the O₂ sensor 17 is placed on the downstream side of thefirst catalyst 15 a. Alternatively, the O₂ sensor 17 may be installed toan intermediate portion of the first catalyst 15 a. In such a case, theO₂ sensor 17 may be installed to the substrate of the first catalyst 15a. In any of the above cases, it is only required that the O₂ sensor 17is constructed to use the exhaust gas after the purification thereof atthe first catalyst 15 a as the sensing subject to sense the gascomponent(s).

Besides the O₂ sensor 17 having the above-described structure, the gassensor may be a gas sensor that has a two-cell structure, which includesan electromotive force cell and a pump cell. In such a case, the outputcharacteristic can be appropriately changed at the electromotive forcecell of the two-cell type gas sensor.

The invention claimed is:
 1. A gas sensor control device applied to anexhaust gas purifying device of an internal combustion engine, whichincludes: a catalyst that is installed in an exhaust device of theinternal combustion engine and purifies NOx, which is a lean componentof an exhaust gas of the internal combustion engine, and a richcomponent of the exhaust gas; and a gas sensor that is installed at alocation, which is in an intermediate portion of the catalyst or on adownstream side of the catalyst, to sense a gas component of the exhaustgas, which serves as a sensing subject, after purification of theexhaust gas with the catalyst, wherein the gas sensor includes anelectromotive force cell, which has a solid electrolyte body and a pairof electrodes, to output an electromotive force signal in response to anair-to-fuel ratio of the exhaust gas, the gas sensor control devicecomprising: a current conduction regulating device that induces a flowof a predetermined constant electric current between the pair ofelectrodes of the electromotive force cell; a current sensingarrangement that senses a current value of an actual electric current,which is conducted through the electromotive force cell; and anabnormality determining unit that determines whether an abnormality ispresent in the current conduction regulating device based on the currentvalue of the actual electric current, which is sensed with the currentsensing arrangement, in a case where the flow of the predeterminedconstant electric current is induced by the current conductionregulating device.
 2. The gas sensor control device according to claim1, further comprising a setting unit that sets a command current value,which is a command value of the electric current to be induced by thecurrent conduction regulating device, wherein the abnormalitydetermining unit determines whether the abnormality is present in thecurrent conduction regulating device based on comparison between thecommand current value, which is set by the setting unit, and the currentvalue, which is sensed with the current sensing arrangement.
 3. The gassensor control device according to claim 2, wherein the setting unitsets the command current value based on an operational state of theinternal combustion engine.
 4. The gas sensor control device accordingto claim 1, wherein the abnormality determining unit determines whetherthe abnormality is present in the current conduction regulating devicebased on the current value, which is sensed with the current sensingarrangement, in a case where a temperature of the electromotive forcecell is equal to or higher than a predetermined temperature.
 5. The gassensor control device according to claim 1, wherein: as a catalyticconversion characteristic of the catalyst, which indicates arelationship between an air-to-fuel ratio and a catalytic conversionefficiency of the catalyst, the catalyst has a second air-to-fuel ratiopoint, which is a point of starting an outflow of the NOx from thecatalyst and is located on a rich side of a first air-to-fuel ratiopoint that forms an equilibrium point for the rich component and oxygen;and the current conduction regulating device induces the flow of thepredetermined constant electric current, which has a current value thatcorresponds to a difference between the first air-to-fuel ratio pointand the second air-to-fuel ratio point at the catalyst.
 6. The gassensor control device according to claim 5, wherein: one of the pair ofelectrodes of the electromotive force cell is a reference sideelectrode, which becomes a positive side at a time of outputting anelectromotive force from the electromotive force cell, and another oneof the pair of electrodes is an exhaust side electrode, which becomes anegative side at the time of outputting the electromotive force from theelectromotive force cell; the current conduction regulating deviceinduces the flow of the predetermined constant electric current, whichhas a current value that is required to shift an equilibrium point of agas reaction around the exhaust side electrode of the electromotiveforce cell to the second air-to-fuel ratio point or an adjacent pointthat is adjacent to the second air-to-fuel ratio point.
 7. The gassensor control device according to claim 6, wherein the currentconduction regulating device induces the flow of the predeterminedconstant electric current, which has the current value that is requiredto shift the equilibrium point of the gas reaction around the exhaustside electrode of the electromotive force cell to a point on a rich sideof the second air-to-fuel ratio point.